The semiconductor industry, as well as other complex nanotechnology process industries, requires very tight tolerances in process control. As dimensions of chip continue to shrink, the tolerance requirements continue to become tighter. Accordingly, new more precise ways of measuring very small dimensions, e.g., on the order of a few nanometers, is desired. At this scale, typical microscopies, such as optical microscopy, or Scanning Electron Microscopy, are not suitable to obtain the desired precision, or to make quick, non-invasive measurements, which are also desirable.
Optical spectroscopic techniques have been presented as a solution. The basic principle of optical spectroscopic techniques is to reflect broadband light from the target, and measure the reflected spectrum. The received signal can be based simply on the reflectance of the light from the sample, or the change in polarization state (Psi, Del) of the light caused by the sample. The spectrum is then modeled to retrieve the geometries or other desired parameters of the illuminated sample.
Applications, commonly classed as “model-based applications”, consist of inferring a certain number of parameters of interest (e.g., thicknesses, critical dimension (CD), side wall angle (SWA), etc) that describes the sample (or “target”). The sample may be measured using Ellipsometry, Reflectometry or other techniques and a theoretical model is used to simulate the spectrum. The model is generated based on several parameters that described the target. Some of these parameters, if not all, are the parameters of interest. Using the described model, a simulated spectrum may be mathematically generated. By fitting the simulated spectrum to the experimental spectrum by adjusting the model parameters, the real value of these parameters can be inferred.